1. Field of the Invention
The present invention relates to a semiconductor device and more specifically relates to a nitride compound semiconductor device requiring high withstand voltage.
2. Description of the Related Art
Group III nitride compound semiconductors have high breakdown voltage. Accordingly, for example, a group III nitride compound semiconductor is applied to a high withstand voltage power device or the like as a field effect transistor (FET) including a source electrode, a drain electrode, a gate electrode, and the like arranged on a principal surface of a crystal conductor composed of a nitride compound semiconductor formed by epitaxial growth or the like. In these nitride compound semiconductor devices, distance between the electrodes, such as between the gate and drain electrodes, needs to be made longer if higher withstand voltage is required. Examples of the group III nitride compound semiconductors include aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN). A representative group III nitride compound semiconductor is expressed by AlxMyGa1-x-yN (0<=x<=1, 0<=y<=1, 0<=x+y<=1). Herein, M is Indium (In), boron (B), or the like.
In the nitride compound semiconductor device, however, while the gate and drain electrodes are negatively biased (the FET is off), on-resistance after the negative bias is applied increases because of the current collapse phenomenon. For example, in an FET, in a state where the FET is on just after high voltage is applied, current flowing between the source and drain electrodes is reduced because of carriers, such as electrons, trapped by the crystal surface between the gate and drain electrodes.
Generally, the following countermeasures are taken to prevent the current collapse phenomenon and increase the withstand voltage:
(1) The crystal surface is coated with an oxide film, a nitride film, or the like for passivation.
(2) A field plate structure is used.
(3) The above (1) and (2) are combined.
(4) A crystal surface including little lattice defects is implemented.
For example, in a FET including gallium arsenic (GaAs), any one of the above countermeasures is employed in some cases as a countermeasure for a problem of frequency dispersion accompanied with the increase in withstand voltage. However, in an FET in which the distance between the gate and drain electrodes is increased for the purpose of increasing the withstand voltage of the device, the effect of the surface level tends to remain even if the passivation coating is employed. In the case of employing the field plate structure, the electric field is concentrated on edge part of a metallic film used as a field plate. The aforementioned countermeasures for withstand voltage then do not produce much improvement.
Especially wide gap compound semiconductor materials of gallium nitride (GaN) which are expected to have high withstand voltage include crystal defects more than silicon (Si) or GaAs, and the current collapse phenomenon thereof is more remarkable. Furthermore, a GaN compound semiconductor device having a Schottky gate structure has large gate leak current.